About 200 US businesses have been hit by a "colossal" ransomware attack, according to a cyber-security firm.
Huntress Labs said the hack targeted Florida-based IT company Kaseya before spreading through corporate networks that use its software.
Kaseya said in a statement on its own website that it was investigating a "potential attack".
Huntress Labs said it believed the Russia-linked REvil ransomware gang was responsible.
The US Cybersecurity and Infrastructure Agency, a federal agency, said in a statement that it was taking action to address the attack.
The cyber-breach emerged on Friday afternoon as companies across the US were clocking off for the long Independence Day weekend.
Kaseya said one of its applications that runs corporate servers, desktop computers and network devices might have been compromised.
The company said it was urging customers that use its VSA tool to immediately shut down their servers.
Kaseya said in its statement that a "small number" of companies had been affected, though Huntress Labs said the number is already about 200 and counting.
It is not clear what specific companies have been affected - a Kaseya representative contacted by the BBC declined to give details.
Kaseya's website says it has a presence in over 10 countries and more than 10,000 customers.
Source: BBC